Is the cable housing to the rear derailleur too short? This condition can restrict the movement of the derailleur pivot at the mounting bolt, nullifying the effect of adjusting the "B" screw. The newer gear cable housing is much less compliant than the old spiral wound housing that was much more like brake cable housing. This will cause problems if cut too short. Unfortunately, I can't give you the correct length for your bike.

Cheers,

Ed